A battery generally includes a positive electrode (cathode during discharge), a negative electrode (anode during discharge) and an electrolyte for ion transport. The electrolyte can contain one or more ionic species that that act as charge carriers. Many widely available battery systems are based on cation electrode reactions, with electrodes capturing or releasing a cation from an electrolyte and balancing the charge with an electron from the external circuit. Because of its very low electrochemical oxidation/reduction potential and light weight, the element lithium is commonly used in cation based battery systems. Both lithium and lithium-ion batteries are commercially available and widely used.
However, the electrochemistry of lithium metal or lithium containing electrodes presents problems for commercial use. Lithium metal is highly reactive, and substantial extra processing may be needed to store lithium in safer intercalate, forms, increasing battery weight and reducing energy density. Li-ion batteries are not stable in many situations, and can be easily overheated or overcharged. In extreme cases, this can result in thermal runaway and battery cell rupture, or short circuiting between the electrodes. For safety and to allow for high cycle lifetime, lithium-ion battery packs often contain expensive voltage and thermal control circuitry to shut down the battery when voltage or temperature is outside a safe range.
Anion based electrode reactions offer one solution to the problems associated with lithium and lithium-ion batteries. In an anion based system, the electrode captures or releases an anion from electrolyte, with concomitant release or capture of an electron from the external circuit. Such anion systems have been used in solid state battery systems, for example, by U.S. Pat. No. 7,722,993 to Potanin, which describes an embodiment of a secondary electrochemical cell where fluoride ions are reversibly exchanged between anode and cathode during charge-discharge cycles, with these electrodes in contact with a solid-state fluoride-conducting electrolyte. Potanin describes solid state electrolytes containing fluorides of La, Ce or the compound fluorides based on them together with an alloying additives, such as fluoride/fluorides of alkaline-earth metals (CaF2, SrF2, BaF2) and/or fluorides of alkaline metals (LiF, KF, NaF) and/or alkaline metal chlorides (LiCl, KCl, NaCl), as well as a wide range of other compound fluorides.
Attempts have also been made to provide anion charge carrier based electrochemical systems capable of using liquid electrolytes. For example, US20100221603A1 “Lithium Ion Fluoride Battery” by Yazami, Darolles, and Weiss disclose a battery including a positive electrode comprising a carbon nanofiber or carbon nanotube material; a negative electrode comprising a graphite material; and an electrolyte provided between the positive electrode and the negative electrode. The electrolyte is selected to conduct charge carriers between the positive electrode and the negative electrode, and includes a solvent-borne fluoride salt is at least partially present in a dissolved state in the electrolyte fluoride ions in the electrolyte. In operation, the positive electrode and negative electrode reversibly exchange fluoride ions with the electrolyte during charging and discharging of the battery. In one embodiment, during discharge of the battery fluoride ions are released from the positive electrode and accommodated by the negative electrode, and/or during charging of the battery fluoride ions are released from the negative electrode and accommodated by the positive electrode. However, for many applications the discussed electrolyte compositions are not lithium-free or do not provide sufficient ion transport capability to ensure reliable, high discharge capacity operation.